1. Field of the Invention
This invention relates to solutions or homogeneous mixtures of an isoimide oligomer and another compound, and is particularly directed to castable or encapsulating resin solutions of an isoimide oligomer and another compound such as a sulfone, the isoimide oligomer and other compound both having functional terminal groups, such as acetylene groups, and being reactive at elevated temperatures to form a copolymer, such solutions having a relatively low melting point and sufficient fluidity at temperatures below curing temperatures to enable the mixture or solution to be used for melt-impregnation or for encapsulating or other applications.
2. Description of the Prior Art
Polyimide resins are materials having excellent thermal mechanical properties at high temperatures, and are also useful in the production of various electrical components because of the good electrical properties of such resins. In order to be formed into useful materials or cured resins, the precursor amic acid or esters are dissolved in an appropriate solvent and processed by conventional casting or dipping techniques. The precursor is then converted to the imide at high processing temperatures through a condensation reaction.
To overcome some of the problems associated with the formation of volatile materials from these condensation reactions, addition curable polyimide oligomers containing acetylene terminal groups and based on the homopolymerization of acetylene have been developed. Representative acetylene substituted polyimide oligomers of this type can be characterized by the structural formula: ##STR1## where R is a divalent organic group, preferably a divalent aryl group, R' is a tetravalent aromatic group, R" is a divalent aryl group, and n is 1 to about 20. Preferably, R, R' and R" are aryl having 6-24 carbon atoms and n is 1 to about 15. Polyimides of this type are described in U.S. Pat. Nos. 3,864,309; 3,845,018; 3,879,349; and 3,928,450, all of which are assigned to the present assignee.
However, the above-described acetylene substituted polyimide oligomers are relatively high melting (200.degree.-220.degree. C.) and have a very narrow "processing window", that is, at temperatures at which they would have good fluidity to be able to be used as a castable resin, the gel time is extremely short. Also, they are soluble in a limited number of solvents such as N-methyl pyrrolidone or N,N-dimethyl formamide. Various attempts have been made to use or incorporate various low melting materials with the above-noted acetylene-substituted polyimide oligomers, to reduce the melting point thereof, but have not been successful due, in large measure, to mutual solubility problems.
An isomeric form of the acetylene-terminated polyimide oligomers has been developed, namely the isoimide form, which has a wide processing window, as for example, a wide solubility range in a large number of available solvents. Hence it is desirable to work with the isoimide form of oligomers and thereafter convert the isoimide form of oligomer to the polyimide form. The isoimide form of oligomer is soluble in a greater number of solvents and also has a lower melting point (140.degree.-160.degree. C.) than the corresponding, e.g. acetylene-substituted, polyimide oligomers. However, even the above-noted isoimide form of the acetylene-terminated oligomers may have too high a melting point to provide a good castable resin.
Acetylene-terminated phenylene resins have been developed in an attempt to produce a resin system which embodies the rheological advantages and specifically the easy processability of epoxy resins and having the high temperature properties of the polyimides. An example of such a material is a resin formed from the oligomer, 4,4'-bis(3-ethynylphenoxy)diphenylsulfone (ATS). Although such an oligomer is quite fluid at low temperatures of 40.degree. C., the cured resin is quite brittle and hence undesirable in this respect. In addition, this latter oligomer does not possess the significant high temperature properties of the copolymers of the present invention.
U.S. Pat. No. 4,098,767 to Bilow, assigned to the present assignee, discloses copolymers of ethynyl terminated polyimide oligomers and a di- or poly ethynyl compound, such as diethynylbenzene. U.S. Pat. No. Re. 30,217 to Bilow, assigned to the present assignee, discloses a copolymer of an acetylene terminated polyimide oligomer and a dinitrile dioxide, such as terephthalonitrile N,N'-dioxide. However, here again, the acetylene terminated polyimide oligomers employed in these latter patents are relatively high melting and copolymerization thereof with the specified monomers requires relatively high curing temperatures.